Flame suppression is typically achieved with the use of both physical and chemical mechanisms. As used herein, "flame suppression" or "suppression" include inhibiting, suppressing, or extinguishing flames. The physical mechanism involves heat absorption by the molecules of a flame extinguishing composition sufficient to lower the temperature of the combusting materials below the ignition point or alternatively by displacement of oxygen, either of which will terminate combustion. Generally, as the extinguishment molecule increases in size (the more atoms and bonds it contains) so do the degrees of vibrational freedom, thus the higher the vapor heat capacity and the greater the heat removal. The chemical mechanism involves interruption of the radical flame-propagation chain reactions, which are the various reactions of hydrogen, oxygen, and hydroxyl radicals that produce flame. It has been speculated that bromine atoms disrupt these chain reactions, which also typically terminates combustion. Halides are ranked for their flame suppression capabilities. Fluorine/fluorides are assigned a value of 1, while the value is 5 for chlorides, 10 for bromides, and 16 for iodides, i.e., iodides are 16 times more effective than fluorine/fluorides.
A variety of agents and techniques are currently used for flame suppression, which use either chemical or physical action, or both, to achieve flame suppression. One conventional agent is a pressurized water extinguisher model that functions solely by thermal energy absorption. Such models are not suitable, however, for use on electrical or flammable-liquid fires. Carbon dioxide (CO.sub.2) and dry-chemical extinguishers are in use and typically displace oxygen and absorb thermal energy, although dry-chemicals typically leave a corrosive residue. Even better for use against grease fires are sodium bicarbonate extinguishers, as well as potassium bicarbonate, urea-based potassium bicarbonate, and potassium chloride extinguishers, although the latter types leave a heavy powdered chemical residue that can damage electrical equipment. Yet another conventional fire extinguisher is the foam (AFFF or FFFP) model, which coats flammable liquids with a chemical to lower the temperature or eliminate oxygen supply, although these are not suitable for electrical fires [Nat'l Fire Protection Ass'n, 1995].
U.S. Army studies on halogenated agents in the 1940's resulted in the adoption of the well known HALON family of flame suppression compositions. HALONS, although environmentally deleterious, are currently in use as highly effective fire suppression agents--particularly in tanks, planes, ships, and heavy engines. Conventional halogenated agents, such as carbon tetrachloride and HALONS, e.g., bromotrifluoromethane, tend to employ both physical and chemical flame suppression mechanisms.
The HALONS are bromofluorocarbons ("BFCs") that are similar to chlorofluorocarbons ("CFCs") but have the formula C.sub.w Br.sub.x Cl.sub.y F.sub.z (where W=1 or 2, Y=0 or 1, and X+Y+Z=2W+2). HALONS must be sufficiently heated and pyrolyzed by a flame to produce free radicals before they achieve sufficient firefighting efficacy. Thus, HALONS are fairly stable and tend to work best on fires with hotter flame temperatures. This stability results in only a 5% effectivity for HALONS. As a result, these organic compounds tend to have long atmospheric lifetimes and migrate to the stratosphere where they undergo photolysis when struck by ultraviolet radiation, thereafter typically decomposing to give chlorine or bromine radicals that act to catalytically destroy the protective ozone layer of the earth, as well as possibly adding to global warming. This depletion of stratospheric ozone allows more ultraviolet light to reach the surface of the earth, resulting in increases in human skin cancer and cataracts, as well as damage to crops, natural ecosystems, and materials and various other adverse effects. Chlorine- and bromine-containing haloalkanes are known to deplete stratospheric ozone, with bromine posing a greater problem (per atom) than chlorine. Indeed, conventional brominated agents and other volatile halogenated alkenes are presently being eliminated from worldwide production, pursuant to the adoption of the Montreal Protocol and the Clean Air Act of 1990, due to their tremendous potential to destroy the stratospheric ozone layer.
The costs of perfluorocarbons are higher, and their firefighting performance is less effective, than those of the brominated agents. In weight and volume critical situations, such as aircraft, tanks, and ships, the additional quantity required for extinguishment is unacceptable. Perfluorinated agents have high global warming potential ("GWP") and atmospheric lifetimes estimated to be several thousand years. Moreover, their production and use is also being restricted by pending legislation and liability concerns of current manufacturers.
In order to quantify these concerns, halogen-containing flame suppression agents are assigned an ozone-depletion potential ("ODP") that reflects their quantitative ability to destroy stratospheric ozone. The ozone depletion potential is calculated in each case relative to CFC-11 (CFCl.sub.3, trichlorofluoromethane), which has been assigned a value of 1.0. Many CFCs have ODPs near 1; HALONS have higher ODPs between 2 and 14, indicating a greater ozone depletion potential. There is thus a need for firefighting, or flame suppression, compositions that overcome the drawbacks of conventional agents as discussed above.
Firefighting compositions to replace HALONS should be effective extinguishants, relatively nontoxic, electrically nonconductive, evaporate cleanly, and have low or no environmental impact. HALONS, although they meet the first four criteria, have long atmospheric lifetimes and high ozone-depletion potentials, and are being phased out of use as discussed above.
Although it is relatively easy to identify fire suppressing agents having one, two, or three of these properties, it is very difficult to identify chemicals that simultaneously possess effective fire suppression performance, non-flammability, low toxicity, cleanliness, electrical non-conductivity, miscibility with common lubricants, short atmosphere and environmental lifetimes, low or no ODP, and very low GWP. Other characteristics are desirable, such as reduced toxicity, which is another major issue in the selection of firefighting agents. For example, the toxic effects of haloalkenes includes simulation or suppression of the central nervous system, initiation of cardiac arrhythmias, and sensitization of the heart to adrenaline. Inhalation of gaseous haloalkanes can cause bronchoconstriction, reduce pulmonary compliance, depress respiratory volume, reduce mean arterial blood pressure, and produce tachycardia. Long term effects can include hepatotoxicity, mutagenesis, teratogenesis, and carcinogenicity.
Furthermore, firefighting agents must also be chemically stable during storage and use over long periods of time, and must be unreactive with the containment system in which they are housed. Firefighting agents must typically be stable on storage at temperatures of about -20.degree. C. to 50.degree. C., and should decompose at flame temperatures to yield radical-trapping species.
A variety of alternative agents containing halides are known for fire suppression, although they are either less effective than HALONS or lack one of the characteristics desired in flame suppression agents as described above. Some of these methods and agents are discussed below. For example, one neat iodinated agent (trifluoroiodomethane, CF.sub.3 I) has long been known to have firefighting potential [Dictionary of Organic Compounds, Chapman and Hall, New York, p. 5477 (1982)].
U.S. Pat. No. 2,136,963 discloses a fire extinguishing agent and method that covers burning material with foam produced by a mixture of a liquid and a compound selected from high molecular quaternary ammonium, phosphonium, and sulphonium compounds. These compounds are disclosed to include a variety of bromides and iodides.
U.S. Pat. No. 2,818,381 discloses methyl bromide used for extinguishing fires. This reference also discloses another early fire extinguishing composition having 10-40 parts by weight of a chloro-difluoro methane having between one and two chlorine atoms, with 90-60 parts by weight of a mixture of bromoform and ethyl bromide.
U.S. Pat. No. 3,779,825 discloses a solid propellant composition having 60 to 90 weight percent oxidizer component selected from solid inorganic oxidizing salts of ammonium perchlorate, the alkali metal perchlorates, ammonium nitrate, the alkali metal nitrates, and mixtures thereof, at least a major portion of the oxidizer being of the perchlorates; from 10 to 40 weight percent of a binder of a rubbery material; and from 0.1 to 8 weight percent of a burning rate depressing agent.
U.S. Pat. No. 4,406,797 discloses a fire extinguishing composition having a mixture of finely divided aluminum compound and an alkali metal, stannous or plumbous halide. The metal halide may include an alkali metal, e.g., potassium iodide, bromide, or chloride, or stannous or plumbous iodide, bromide or chloride, although potassium iodide is disclosed to be preferred for use in the composition.
U.S. Pat. Nos. 4,486,317 and 4,380,482 disclose methods for preparing and compositions of a thickener in aqueous solution or slurry stabilized against thermal degradation by inclusion of any of a variety of suitable iodide and/or iodate ions in the solution or slurry. The method and composition also include a thermally stabilizing amount of iodide ion selected from hydriodic acid, ammonium iodide, an alkyl-substituted ammonium iodide, or an alkali metal or alkaline-earth metal iodide, iodate ion selected from iodic acid, ammonium iodate, an alkyl-substituted ammonium iodate, or an alkali metal or alkaline-earth metal iodate, or a combination of the iodide and iodate ion compound. The iodate ion is disclosed as effective in amounts up to about 0.6%, although an iodate concentration up to 0.3% is preferred, as higher concentrations result in conversion of iodate to iodide over time and high temperature.
U.S. Pat. Nos. 4,961,865 and 4,950,410 disclose methods and compositions for inhibiting the combustion of wood and other cellulosic materials by impregnating such material with compositions including a mixture of sodium chloride, magnesium chloride, sodium sulfate, sodium borate, calcium chloride, magnesium sulfate, and water. The composition may also optionally include calcium sulfate, potassium sulfate, calcium chloride, magnesium sulfate, magnesium bromide, and potassium chloride.
U.S. Pat. No. 5,466,386 discloses fire-extinguishing compositions of low ozone depletion potential having dry particles of ammonium bromide coated with a water repelling, solid, non-flammable adherent, such as zinc stearate, to improve flowability. The particles allegedly enhance the fire-extinguishing properties of chlorofluorocarbons and halogenated paraffins having low ozone depletion properties when dispersed therein.
U.S. Pat. No. 5,520,826 discloses a flame extinguishing pyrotechnic having an azido binder, such as a glycidyl azide polymer (GAP), an azido plasticizer, a solid tetrazole, and a perfluorocarboxylic acid salt cured to a rubbery composite by the addition of an isocyanate that flamelessly deflagrates to produce primarily nitrogen, carbon dioxide, and a fluoroolefin.
U.S. Pat. No. 5,562,861 discloses a set of environmentally safe, nonflammable, low-toxicity refrigerants, solvents, foam blowing agents, propellants, and firefighting agents that allegedly have no ozone-depletion potential. These agents include at least one fluoroiodocarbon agent of the formula C.sub.a H.sub.b Br.sub.c Cl.sub.d F.sub.e I.sub.f N.sub.g O.sub.h, where a is 1 to 8; b is 0 to 2; c, d, g, and h are each 0 to 1; e is 1 to 18; and f is 1 to 2. This reference also notes that conventional chemical wisdom indicates that iodine-containing organic compounds are too toxic and unstable to use for these purposes, and iodocarbons have been rejected on those grounds by the majority of those skilled in the art.
U.S. Pat. No. 5,626,786 discloses a class of fire suppressant compounds having labile bromine atoms bound to non-carbon atoms that are alleged to be more effective than HALON 1211 and 1301 at suppressing fires. These compounds are disclosed to hydrolyze or oxidize rapidly in the troposphere, thereby having minimal ODP.
Reduction of toxicity, ODP, and other environmental effects must be balanced against effective flame suppression to achieve a superior flame suppression composition and method. Although more recent conventional flame suppression compositions have achieved limited ozone depletion potential, it is typically at the expense of fire suppression effectiveness or volume efficiency. Thus, the need exists for an environmentally-friendly, non-toxic fire suppression composition, and use thereof, that have better fire suppression effectiveness than HALON-type agents.